1. Field of the Invention
The present invention concerns an improvement in a magnetic field generation device using superconductive coils used, for example, in medical nuclear magnetic resonance tomographic device (hereinafter referred to as MRI), and more particularly, it relates to a magnetic field generation device used for superconductive type MRI which is relatively inexpensive and capable of forming a magnetic field having a high uniformity in a predetermined space required for medical use.
2. Description of the Prior Art
MRI is a device in which a person to be inspected is partially or wholly inserted into a gap of a magnetic generation device forming an intense magnetic field to obtain a tomographic image of an object and which can draw out as far as the nature of the tissues thereof.
The magnetic field generation device for use in MRI includes a permanent magnet type, a normally conductive type and a superconductive type, among which the superconductive type has been used generally so far since an extremely high magnetic field of 0.5-2 T(Tesla) can be obtained stably.
In the superconductive type MRI, since a person to be inspected has to be inserted in an intense static magnetic field, cylindrical coils such as solenoid coils or Hemlholtz's coils are usually formed and the person to be inspected is inserted into a cylindrical space defined by the coils and the axial direction of the cylindrical space aligns with a body axis of the person to be inspected.
The coils of the aforementioned construction entails the problem that opening portions of the device are small and disposed only at two positions, and the required uniformity of magnetic field cannot be obtained unless the length along the depth of the device is extended for taking images of high quality, so that a psychological oppressive sensation is caused to the person to be inserted as the length in the direction of the depth of the device is increased.
As a construction of relatively reduced length along the direction of the depth of the devices compared with the construction of using the cylindrical coils described above, a superconductive type MRI has been proposed in which a pair of magnet pole pieces having superconductive coils disposed circumferentially are opposed to each other to define a predetermined gap therebetween See U.S. Pat. No. 4,766,378. However, also in this construction, since a pair of magnet pole pieces are disposed on a disc-like yoke disposed on the both ends of a short cylindrical yoke and a person to be inspected is inserted from an opening formed to the outer circumference of the short cylindrical yoke as shown in FIG. 14, no substantial reduction in the psychological oppressive sensation is obtained.
In either of the constructions using the cylindrical coils or a pair of magnet pole pieces as described above, it has been keenly demanded to reduce the psychological oppressive sensation to the person to be inspected. Furthermore, since superconductive coils are used as a magnetic field generation source, it is indispensable to provide a cooling device for keeping the coils, for example, to a temperature of liquid helium, i.e., a cryogenic temperature, so that effective arrangement and construction of the superconductive coils has keenly been demanded in order to reduce the cost of the superconductive type MRI.
However, in the superconductive type MRI using, for example, a pair of magnetic pole pieces as described above, since a pair of expensive superconductive coils are necessary and if a single-side excitation system is adopted in which superconductive coils are disposed circumferentially to only one of the opposed paired coils, merely with an aim of reducing the cost, the magnetic field is increased on the side of the magnetic pole piece having the superconductive coils disposed circumferentially and, accordingly, imbalance of the magnetic field occurs in the vertical direction the magnetic field uniformity in the predetermined space in which the person to be inserted is worsened extremely.
It is an object of the present invention to provide a magnetic field generation device for use in a superconductive type MRI using a relatively inexpensive construction in which superconductive coils are disposed circumferentially to only one of a pair of magnet pole pieces opposed to each other, and which is capable of satisfying a required high magnetic field uniformity for medical use and has an open construction with such a large opening as not giving a psychological oppressive sensation to a person to be inspected.